The present invention relates to an optical switch for making light in an optical waveguide selectively emergent therefrom, and a display unit on which the optical switches are arrayed.
In home televisions, a cathode-ray tube having a mechanism of emitting light by exciting phosphors with electron beams is used as a display. In liquid crystal displays, a light transmittance is changed by varying a polarization characteristic of liquid crystal. In these liquid crystal displays, a color of white light is selected by using a filter. In plasma displays, phosphors are excited with ultraviolet rays generated by plasma.
By the way, television receivers have disadvantages that a depth of a cathode-ray tube is long, thereby making it impossible to realize a thin display, and that the weight of the cathode-ray tube is heavy. A further disadvantage of the television receivers is that since light emission is obtained by exciting phosphors, a half-width of an emission spectrum of each of three primary colors is large, to degrade a color purity and a color reproducing characteristic. Liquid crystal displays have a disadvantage that since a half-width of an emission spectrum determined by a color filter is also large, to degrade a color purity and a color reproducing characteristic. Plasma displays have disadvantages that since light emission is obtained by exciting phosphors like cathode-ray tubes, a half-width of each emission spectrum is large, to degrade a color purity and a color reproducing characteristic, and that it is not easy to adjust gradation of an image.
On the other hand, as display units utilizing photonics, there are known display units using optical waveguides. Such a display unit, however, has a problem that a contrast ratio of light emergent in response to turn-on/turn-off of an optical switching device, that is, an optical switch such as liquid crystal is low. Further, an optical switch having a structure in which light transmissive layers are stacked has another problem that a slight change in light emergence efficiency depending on a thickness and a refractive index of each layer of the stacked structure may exert a large effect on an uniformity of the entire light emergence efficiency, and therefore, it is expected to provide an optical switch capable of easily realizing the uniformity of a light emergence efficiency.
An optical switch composed of an optical waveguide including at least a cladding layer, and a light directivity coupler having an electrode film, an alignment control film, and ferroelectric liquid crystal filled between a pair of substrates is known, for example, from Japanese Patent Laid-open No. Hei 8-36196. The design of this optical switch aims that a coupling efficiency (light emergence efficiency) becomes 1, that is, a transfer rate of light becomes 100% by optimizing a refractive index of liquid crystal, and with respect to such design of the optical switch, the above document describes that the coupling efficiency can reach 98% by setting an effective refractive index of liquid crystal to 1.523.
An optical switch designed to pursue a high coupling efficiency as the optical switch described in the above document, however, has a problem. Namely, a refractive index of each component such as ferroelectric liquid crystal, an optical waveguide, an electrode film, or an alignment control film may be deviated from a design value due to variations which occur depending on a thickness and a material characteristic of each layer in production steps, and if the refractive index of a component is deviated from a design value, then such a deviation cannot be canceled only by adjusting a refractive index of ferroelectric liquid crystal, and the coupling efficiency is largely degraded as the deviation in the refractive index of the component from the design value becomes large, thereby failing to obtain the uniformity of a light emergence efficiency.